Multimode fibers are used for short-distance applications and local networks. The core of a multimode fiber generally has a diameter of approximately 50 microns (μm), compared with approximately 8 to 9 microns (μm) for the core of a single-mode fiber. Thus, for a particular wavelength, several optical modes propagate simultaneously along the fiber, carrying the same information. The bandwidth is directly linked to the group velocity of the optical modes propagating in the multimode core of the fiber. To guarantee a high bandwidth, it is necessary for the group velocities of all the modes to be identical. In other words, the intermodal dispersion (i.e., the difference in group velocity between the slower mode and the faster mode) should be minimized for a particular wavelength. The multimode fibers have been the subject of international standardization under standard ITU-T G.651, which, in particular, defines criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility.
To reduce the intermodal dispersion in a multimode fiber, it has been proposed since the 1970s to produce graded index fibers with a parabolic core profile. Such a fiber has been used for many years and its characteristics have been described in particular in the publications “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell System Technical Journal 1973, pp. 1563-1578, and “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177.
A graded-index profile can be defined by a relationship between the index value n at a point as a function of the distance r from this point to the center of the fiber:
  n  =            n      1        ⁢                  1        -                  2          ⁢                                    Δ              ⁡                              (                                  r                  a                                )                                      α                                              wherein,        α>0 (α→∞ corresponding to a step index profile);        n1 is the maximum index of the multimode core;        a is the radius of the multimode core; and        
  Δ  =            (                        n          1          2                -                  n          0          2                    )              2      ⁢              n        1        2                            wherein,        n0 is the minimum index of the multimode core, generally corresponding to the index of the cladding (most often made of silica).        
A multimode fiber with a graded index therefore has a core profile with a rotational symmetry such that along any radial direction the value of the index decreases continuously from the center of the fiber to its periphery. These curves are generally representative of the theoretical or target profile of the optical fiber, though fiber-manufacturing constraints may lead to a slightly different profile.
When a light signal propagates in such a core having a graded index, the different modes experience a different propagation medium, which affects their speed of propagation differently. By adjusting the value of the parameter α, it is therefore possible to theoretically obtain a group velocity that is virtually equal for all the modes and thus a reduced intermodal dispersion for a particular wavelength. A value for the parameter α of between 1.8 and 2.2 generally allows a satisfactory limitation of the modal dispersion.
That said, an optimum value of the parameter α is valid only for a particular wavelength. Thus, a multimode fiber typically transmits a monochromatic optical signal having a particular wavelength for which the alpha profile of the fiber has been optimized.
To date, high-bitrate transmission Ethernet networks are in operation, with bitrates of the order of 10 GbE (10 Gb/s). In order to provide such bitrates over more than 300 meters and 550 meters, respectively, it is necessary to guarantee an effective bandwidth greater than or equal to 2000 MHz-km and 4700 MHz-km, respectively. The standard TIA-492AAAC-A standardizes the required performances for 50 μm-diameter high-bitrate multimode fibers. However, the effective bandwidth (denoted by the acronym EMB for “Effective Modal Bandwidth”) depends on the source used.
In a manner known per se, the effective bandwidth EMB is determined by a measurement of the delay caused by the modal dispersion, known as “Dispersion Mode Delay” (DMD) graphical representation. The procedure for measuring the DMD is the subject of standardization (IEC 60793-1-49 and FOTP-220).
A DMD graphical representation is obtained by injecting a light pulse having a particular wavelength λ0 at the center of the fiber and by measuring the pulse delay after a particular fiber length L. The introduction of the light pulse of particular wavelength λ0 is radially offset to cover the entire core of the multimode fiber. When the parameter α is set to an optimum value (αoptimum), there is virtually no shift in the light pulse delay for a particular wavelength λ0 regardless of the injection point of the pulse along the radius r of the fiber core; the intermodal dispersion is low and the effective bandwidth high.
However, this alignment on the DMD graphical representation of the light pulse delays regardless of the radius r, are only valid for a particular wavelength λ0 for a particular value of the parameter α, αoptimum. A multimode fiber is thus typically optimized to transmit a signal propagating at an optimum wavelength (λoptimum). When a different wavelength pulse is transmitted in this same multimode fiber, the modal dispersion can become significant and restrict the bandwidth below the value of 2000 MHz-km required by the current standards.
There is thus a need for multimode telecommunication networks having bitrates greater than 10 GbE. It is sought to achieve bitrates of 40 GbE or even 100 GbE. It is however difficult to reach such bitrates with a single transmission channel. Nevertheless, for the foregoing reasons expressed, wavelength multiplexing is not possible directly in a multimode fiber.
Wavelength multiplexing, WDM for “Wavelength Division Multiplexing,” consists of transmitting several light pulses of different wavelengths on a single optical fiber while combining them on input using a multiplexer (MUX) and separating them on output using a demultiplexer (DEMUX). Typically, dense multiplexing systems, DWDM for “Dense Wavelength Division Multiplexing,” are used with single-mode fibers for which modal dispersion is non-existent, only the chromatic dispersion requiring compensation.
Thus, a wavelength multiplexing in a multimode fiber requires not only a compensation for the chromatic dispersion, but also a management of the modal dispersion.
The publication “10×10 Gb/s DWDM Transmission through 2.2-km Multimode Fiber Using Adaptive Optics” by R. A. Panicker et al., IEEE Photonics Technology Letters, Vol. 19, No. 15, pp. 1154-1156 published Aug. 1, 2007, proposes a wavelength multiplexing in a multimode optical fiber. An adaptive optics system is provided at the fiber input to shape the signal and minimize the modal dispersion. The adaptive optics system is however complex and costly, and designed to operate in the C band while most multimode applications are located around 850 nanometers.
U.S. Pat. No. 7,242,870 describes a WDM system comprising a multimode optical fiber transmitting a wavelength multiplexed signal. The multimode fiber is germanium-fluorine co-doped and has a controlled index profile in order to maximize the bandwidth in the 720 nanometer to 1400 nanometer transmission window. The profile and concentrations of dopants for producing such a fiber, however, are difficult to control and the cost of the fiber is thereby increased.
U.S. Pat. No. 6,525,853 describes a communication system in which N optical signals are combined for transmission in a single multimode fiber. An optical system is provided at the multimode fiber input and introduces a modal coupling diversity which, when combined with the modal dispersion of the fiber, introduces a decorrelation of the signals transmitted and received. This decorrelation makes it possible to retrieve the data flow of each signal transmitted by means of a suitable algorithm. However, the system described in this document is complex to implement.
U.S. Pat. No. 5,278,687 describes a bidirectional optical transmission system comprising a multimode optical fiber transmitting a wavelength multiplexed signal. The modal dispersion is simply ignored, which is prejudicial for the system bandwidth.
U.S. Pat. No. 6,363,195 proposes to compensate for the modal dispersion of a multimode optical link by using a concatenation of multimode fibers in order to optimize the bandwidth for two transmission windows, one centered on 850 nanometers and the other on 1300 nanometers. This document proposes to use a length of a first multimode fiber having a value of parameter α1 of between 0.8 and 2.1 to optimize the bandwidth at 850 nanometers and a length of a second multimode fiber having a value of parameter α2 between the first value α1 and 8 to optimize the bandwidth at 1300 nanometers. This document, however, makes no mention of a wavelength multiplexing.
Thus, there is a need for a multimode optical transmission system allowing a dense wavelength multiplexing (DWDM) for an increase in bitrate without reducing the bandwidth. Such an optical system must be uncomplicated and efficient, and must be implemented with a standard multimode transmission fiber.